Since you bought your Mac mini with Bluetooth, you could use a Sprint Treo 650 smartphone (with Bluetooth) to add an always-on Internet connection. We do it all the time with a Powerbook so I can tell you it works great even at 70 MPH. Here's a link to TreoCentral that'll tell you everything you need to know to get up and running:
http://discussion.treocentral.com/showthread.php?t=63250.

Since you bought your Mac mini with Bluetooth, you could use a Sprint Treo 650 smartphone (with Bluetooth) to add an always-on Internet connection. We do it all the time with a Powerbook so I can tell you it works great even at 70 MPH. Here's a link to TreoCentral that'll tell you everything you need to know to get up and running:
http://discussion.treocentral.com/showthread.php?t=63250.

I use the Treo 650 unlocked GSM version with T-mobile currently._________________Dennis M.

I've got a Discovery Series I (1998) and I've been meaning to do this for a while, in order to hook up Topo! and the navigation GPS. A must-have, in my opinion.

Besides, it's a Land Rover, and you didn't tell us how well the system works off-road. This isn't a "pavement queen" is it? How about hooking in a satellite phone for backcountry web access (shudder). How about an undercarriage-mounted iSight for checking rock clearance? Foreign-language translation software, for operating in Zimbabwe?

I'm just kidding, of course. Mostly. I do use Topo! and my GPS with my iBook, which I wish I could find a decent mount that would fit in the front of the Rover.

Cool project but I have a suggestion for the owner concerning the power problem. The setup described is going from 12 (car) to 120V inverter to battery BU which is (120--> 12) internally and then back up to(12--> 120) and then back down with the Mac Mini power supply to low voltage (120--> mm). Too many voltage changes of which each has inefficiencies. Why not connect the car's 12 volt system directly to the 12 battery in the BU unit and loose the inverter. The battery BU unit would have to be opened to access the battery and a large current (25 amp or so) diode connected in series with the 12 from the car to the BU unit would be a good idea ( http://www.allelectronics.com/cgi-bin/category.cgi?category=205&item=1N1190AR&type=store ). This prevents the BU battery from discharging into or interfering with the car supply yet allows the car to keep the BU battery hot and charge it quickly.

A USB compatible UPS (Uninterruptible Power Supply) would provide the means to sleep the Mac Mini when you leave the car. Mac OS X has built in support for this (if it detects one it shows the option in Energy Saver). However I do note your problem getting a UPS to properly charge via a 12V to 110V power inverter.